After a pomp-filled welcome in Ankara where his NATO allies were at pains to keep him onside, US President Donald Trump was to get down to the hard business of the summit on Wednesday.

As he landed in the Turkish capital, Trump was effusive in his praise for President Recep Tayyip Erdogan, highlighting their "chemistry" in remarks that contrasted sharply with his lingering bitterness with European allies over the war on Iran.

"I was very disappointed with NATO," Trump repeated, setting the tone for the summit's main gathering where the alliance's 31 other leaders will come face-to-face with the mercurial US leader when the main session opens at 0815 GMT.

The summit comes at a fraught time for the 77-year-old transatlantic alliance, with Trump demanding members make good on a pledge to ramp up defence spending as Washington takes a step back from Europe.

On the eve of the key session, the alliance published figures showing core defence spending by Europe had risen by 11 percent in 2026 and would hit $634 billion, up from $571 billion a year earlier.

Keen to avoid a new confrontation with Trump, NATO allies unveiled tens of billions in new arms contracts on Tuesday in a bid to prove they were making good on a pledge to hike defence spending.

European allies were "delivering" by bolstering military budgets and moving to take more responsibility for the defence of their continent in the face of Russia, insisted NATO chief Mark Rutte.

- Russia, Ukraine 'want deal' -

With NATO keen to focus the US leader's attention on its surging defence budgets, stalled efforts to halt the war in Ukraine were also back on the agenda, with Trump saying he believed both sides wanted to end the fighting.

"I think they both want to make a deal," said Trump, ahead of talks on Wednesday with Ukraine's Volodymyr Zelensky on the sidelines of the summit.

With negotiations at an impasse, Trump spoke with Russia's Vladimir Putin before flying to Turkey and was to "follow up" with him after meeting Zelensky, a US official said.

Europe and Canada are set to pledge to keep military support flowing to Ukraine to the tune of 70 billion euros ($80 billion) a year in both 2026 and 2027.

But Zelensky urged a NATO defence forum on Tuesday to step up help for Ukraine's air defences as it struggles with shortages of crucial interceptors, and made a new pitch for Kyiv joining the alliance.

Trump will also hold talks with Syrian President Ahmed al-Sharaa as he seeks to rebuild the country's international image as it emerges from years of civil war.

The meeting comes a day after the Syrian leader hosted French President Emmanuel Macron for a landmark state visit that was overshadowed by twin bomb attacks that wounded 18 in the capital Damascus.

- 'No sanctions for friends' -

Although Trump still appears rankled over restrictions some allies put on US forces using bases at the start of the Iran conflict, he brought welcome news for Erdogan, saying he would consider selling Turkey F-35 fighter jets and would roll back sanctions.

Ankara was booted out of the F-35 programme in 2019 after buying a Russian defence system, and has long sought a way to regain admittance and to secure the lifting of US sanctions that have soured ties and hampered Turkish defence projects.

"We're going to be taking the sanctions off," Trump said at Erdogan's opulent marble-clad presidential palace. "We don't want to sanction friends."

Sitting next to him, Erdogan said he was confident Trump would end the dispute.